HackDatKiwiCTF 2015 - Vigenere 1 / 2

Vigenere 1 / 2

Crypto - 50/80 Points

Vigenere is a classic cryptographic cipher. It is very simple, yet not that easy to break. Unless you break Vigenere on your own, you can’t call yourself a cryptanalyst. Start with the simplest mode, Chosen Plaintext Attack:

So you got crypto skills, right? Try it again on Known Plaintext Mode:

Writeup

In Vigenere 1 we had a Chosen Plaintext, where you give a plain text input and the webpage return the ciphertext.
The solution is very simple. In the default Vigenere alphabet, “A” is the Reflecting character so a plaintext of
“AAAAAAAAAA” returns the key as ciphertext.

The key was “KIWIKI”, it was repeated but that’s how Vigenere so you take the shortest non-repeating key.

In Vigenere 2 we had a Known Plaintext, where you know both plaintext and it’s ciphertext.
You can do this by hands but I made a simple script that can do this for you.